In constructing a light beam scanning system, there may be a need to combine multiple light beams to: achieve a high power density scan exposure of a medium, increase throughput by writing sequential lines at higher scan rate, write plural lines simultaneously, or provide output beams of selectable power (such as the provision of a preheating or erase scanning beam at a leading scan position, followed by a write beam at a following, or lagging, scan position).
These applications have heretofore required the use of atypical high power laser beam source to produce an output beam of higher power density than achievable by common beam sources. For example, image and/or information signal recording media such as dye transfer or phase change media require on the order of one million ergs/.sup.2 for exposure. A compact laser source offering power levels from 100 mW to 1 W is favored for such applications; however, such devices, if available, are costly to manufacture and difficult to use because the task of modulating such a source over a large range of output values induces the laser source to mode hop (change wavelength). Another drawback is that any associated optical systems that rely on diffraction effects (such as a hologon beam deflector) will suffer from undesireable mode hop artifacts, such as an angular shift in the scanning beam. In a laser hologon-based printer, for example, mode hop causes the exposure of illegible characters.
It is known in the optical art to provide the alignment of plural input beams into an output beam by use of a beam splitter operated in a reversed orientation.
However, the beam splitters proposed heretofore for beam combination, such as multi-layer dielectric coated mirrors, grating element, and pellicle beam splitters, exhibit a host of drawbacks. The named beam combiners are inefficient (i.e., they exhibit an undesireable amount of beam power loss) and are undesireably sensitive to factors such as the separation angle of the input beams and the ambient temperature. These drawbacks would be especially disadvantageous when the beam combiner is operated in a scanner wherein the radiometric accuracy of the output beam must be accurately maintained.
For example, as proposed by Tateoka in U.S. Pat. No. 4,634,232, two glass triangular prisms, one of which is coated with a polarizing multilayer mirror coating, are cemented together so that the mirror surface is on the cube diagonal. The multilayer mirror is typically a series of quarter wave interference coatings. The mirror surface transmits a first beam of one polarization and reflects another beam polarized orthogonally to the first beam. The passband, and hence the reflectivity and transmissivity of the mirror, is temperature, wavelength, and angle sensitive. Typically, only 80% of P-polarized light is transmitted and 90% of S-polarized light is reflected. Furthermore, the polarization purity of each beam component in the resulting output beam is unacceptable for some applications. The leakage component causes interference and the resultant output beam intensity fluctuations can be quite objectionable.
Beam combiners formed from certain birefringent elements have been proposed for single wavelength multiple beam combiners. For example, a beam combiner in the form of a Wollaston or Rochon prism was proposed by Tatsuno et al. in U.S. Pat. No. 4,822,151 to combine phase-locked input beams emanating from a diode laser array operating at a single wavelength. However, such a beam combiner is more complicated than desirable to construct, and if there are errors in manufacturing, the beam combiner will not provide adequate beam overlap.